1. Field of the Invention
The invention relates to compositions which utilize a combination of resins and thermally-conductive particles to produce a composition which resists corrosion and has an enhanced thermal conductivity. The composites can be anisotropic; that is, they exhibit enhanced thermal conductivity in specific directions through the material. The invention also includes methods of making the compositions, and methods of utilizing the compositions.
The invention finds ready application in the field of heat exchangers and heat transfer.
2. Description of Related Art
Heat exchangers promote the transfer of thermal energy (heat) from one medium to another. Depending on the application and the relative temperatures of the media, heat can be transferred in either direction across the heat exchanger.
In the abstract, a common configuration for heat exchangers involves a thermal conductor separating two mediums. Heat from the warmer medium is transferred across the conductor to the cooler medium. In this way, heat is exchanged.
In practice, heat exchangers most often comprise complicated arrangements of metallic tubes soldered together into a continuous path. The manufacture of such complicated piping arrangements is difficult and leaks at faulty solder connections are common. Furthermore, the solder connections loosen from the continuous vibrational environment in which many heat exchangers are used. In some heat exchangers, the metallic tubes are surrounded by metal fins in an attempt to make the most efficient use of available heat conducting surfaces. These metallic fins are themselves difficult to manufacture and are relatively inefficient. In other heat exchanger applications, the metallic tubes are wrapped into a coil and placed in contact with the outer surface of a container. In such arrangements, the area of contact between the tubing and the container is relatively small, resulting in an inefficient heat exchanger.
The materials used in existing heat exchangers are generally homogeneous. In other words, the materials are made of one substance having common characteristics throughout. Because the materials are homogeneous, the materials tend to conduct heat equally in all directions (xe2x80x9cisotropicallyxe2x80x9d).
Metal is the most common material from which heat exchangers are built. Generally, metals are good thermal conductors; that is, they have high indexes of thermal conductivity. The best conductors are the xe2x80x9cnoble metalsxe2x80x9d which include gold, silver, and platinum. However, noble metals are very expensive. For most applications, with the notable exception of the aerospace industry, the noble metals are too expensive for practical use as a material in heat exchangers. Even the non-noble metals are relatively expensive materials compared to resins.
Metals have other qualities beyond price that limit their use in heat exchangers. In a typical metal heat exchanger, pieces of metal are connected to each other by brazing. These heat exchangers contain a large number of joints, each of which must be brazed. Brazing is expensive, and the joints formed by brazing often leak.
Metal heat exchangers are difficult to shape. While the metals themselves are malleable, the joints used in the heat exchanger are not malleable, and often break when bent. Metals require extreme temperatures to melt. This makes metals difficult and costly to mold.
Metal heat exchangers are susceptible to corrosion. If the heat exchanger is located in a corrosive environment, most will corrode and eventually fail. A specific example of a corrosive application is a swimming-pool heater or heat pump that is exposed to chlorine. Other corrosive environments are sea water and smoke stacks.
Resins are an alternative material from which heat exchangers can be formed. Resins have the benefit of being easily molded into practically any shape. Chemical resistant resins are well known and can be chosen when required by the application. Resins without more are often brittle and, therefore, may shatter. A heat exchanger made of pure resin may break when exposed to mechanical forces. Also, resins are less thermally conductive than metals. In materials having a low thermal conductivity, the rate at which heat is transferred across a surface (i.e. from one medium to another) is low. For this reason, heat exchangers made of traditional resins have to be larger than heat exchangers of equal capacity made of metal.
Carbon as a sole ingredient is not a useful material from which to build heat exchangers despite its many positive attributes. Carbon does have a higher thermal conductivity than most resins. However, carbon has a lower thermal conductivity than most metals. Carbon is relatively inexpensive, and is commercially available in several grades. Graphite is one grade of structured carbon. Graphite has a higher thermal conductivity but costs more than ordinary carbon. Carbon and graphite comprise particles of various shapes and sizes. These shapes include powder, flakes, and fibers. Carbon is corrosion resistant, but as a sole ingredient cannot be used to make a heat exchanger because carbon is difficult to shape in large homogenous blocks such as those needed to form a heat exchanger. Also, carbon without more is too brittle and susceptible to chipping to be the only ingredient in a heat exchanger.
The prior art discloses adding metal powders to resins to increase thermal conductivity of a composition. Furthermore, the prior art shows specific applications of these materials to heat exchangers. Abbey, UK Patent 558,124, issued Oct. 11, 1944, describes xe2x80x9cImprovements Relating to Heat Exchangers and Like Apparatus Having Heat Radiating Fins or Plates.xe2x80x9d Zygiel, U.S. Pat. No. 3,498,371, issued Mar. 3, 1970, describes a xe2x80x9cHeat Transfer Device.xe2x80x9d The resin-metal compositions disclosed therein are costly because the cost of the metal ingredient is relatively high compared to other materials. The metal in the resin is also susceptible to corrosion. Moreover, the disclosed compositions without more are not able to conduct heat at a higher rate in selected directions through the material (i.e. they are limited to isotropic conduction of heat).
Other patents disclose the addition of graphite powder to resin to make heat exchangers. Dainippon Ink Kagaku Kogyo K. K., JP Patent 60-200097, issued Oct. 9, 1985, describes a xe2x80x9cHeat Exchanger of Hollow Fiber Type.xe2x80x9d Dainippon discloses a heat exchanger utilizing hollow tubes filled with graphite that conduct heat. Dainippon does not disclose a composition that can be molded or shaped.
Hahn, DE Patent 26 37 511, issued Feb. 23, 1978, describes an xe2x80x9cImproved Conductivity Plastic Heat Exchanger Materialxe2x80x94Having Particles of Metal or other High Conductivity Material Embedded in It.xe2x80x9d Hahn discloses in general terms the use of higher thermal-conductivity materials such as metal in a resin. Hahn does not specifically consider carbon or the advantages of carbon over mere highly-conductive particles. Hahn discloses that the particles can be powders, granules, or splinters. Hahn arranges the material into grids, lattices, and veneers that can be located along the exterior to reinforce the resin. Hahn does not disclose a method to integrate the particles into the resin. Hahn does not consider methods involving molds to make compositions having particles arranged to increase thermal conductivity in parts of the resin or in certain preferred directions in the composition.
Plate-type heat exchangers are described in the prior art. Schon, U.S. Pat. No. 4,744,414, issued May 17, 1988, describes a xe2x80x9cPlastic Film Plate-Type Heat Exchanger.xe2x80x9d These heat exchangers comprise a stack of alternating plates sandwiched to each other. Two media (usually liquids) are separated by passing one medium through a first of the alternating plates while passing the other medium through the other set of alternating plates. Heat is transferred from medium to medium through the intervening walls of the plates. Plastic is used to construct the plates. However, as Schon discusses, plastic without more is a poor thermal conductor. The poor conductance of plastic is why Schon requires the use of polymer film to separate the media. Schon does not disclose the inclusion of materials to enhance the thermal conductivity of the resin. Nor does Schon disclose aligning the direction of enhanced thermal conductivity to increase the overall heat transferred by the heat exchanger.
Liquid crystal polymers (xe2x80x9cLCPsxe2x80x9d) are well-known in the prior art. LCPs exhibit anisotropy in the liquid phase. They may be characterized as thermotropic (i.e., liquid crystal in the melt) or lyotropic (i.e., liquid crystal in solution). Liquid crystal polymers have very stiff, rod-like molecules. In the quiescent state, the molecules line up in an ordered array in local regions, thereby forming domains. The individual domains, however, are not lined up into any particular ordered array; instead they exhibit random orientations. Ide discloses a xe2x80x9cProcess for Extruding Liquid Crystal Polymerxe2x80x9d in U.S. Pat. No. 4,332,759, issued on Jun. 1, 1982. Ide discloses a method of shaping LCP through extrusion to make articles having enhanced physical properties due to the orientation of the crystalline domains in the polymer molecules parallel to the flow direction. Ide""s method also produced articles wherein the LCP molecules are oriented in such a manner so that they are self-reinforcing and have mechanical properties comparable to articles formed from fiber-reinforced isotropic polymeric materials. Ide does not consider extruding LCP containing particles. Nor does Ide contemplate or disclose the desirability of creating components with carbonaceous materials to create items having enhanced thermal conductivity. xe2x80x9cCarbonaceous materialsxe2x80x9d can be defined as elements of carbon structured in forms such as graphite and diamonds.
The injection molding of materials comprising 45-60% graphite powder and 40-55% LCP resin for use as an electrically conductive material is disclosed in U.S. Pat. No. 5,882,570, issued on Mar. 16, 1999, to Hayward. This patent describes material optimized for electrical conductivity. For this reason, the material is limited to compositions having 45-60% graphite powder and 40-55% LCP resin. The patent does not describe formulations optimized for thermal conductivity.
Therefore, a need exists for a heat exchanger which is lightweight and inexpensive but which possesses corrosion resistance and heat transfer characteristics similar or superior to those already known.
One object of the invention is to provide a chemical composition comprising a resin with particles encapsulated therein: the particles generally having higher thermal conductivity than the resin. The composition created has a thermal conductivity greater than the thermal conductivity of the resin and the thermal conductivity of the particles alone.
xe2x80x9cThermal conductivityxe2x80x9d is defined as the heat flow across a surface per unit area per unit time, divided by the negative of the rate of change of temperature with distance in a direction perpendicular to the surface; thermal conductivity is also known as heat conductivity. Alternatively, thermal conductivity can generally be thought of as the rate at which heat is conducted through a substance. That term, as used herein refers to either or both concepts.
The invention also encompasses articles that require heat to be directionally transferred throughout themselves.
Another object of the invention is to provide a method of making a composition resulting in anisotropic articles that conduct heat at different rates into, out of, and through the article depending on the design of the article.
A still further object of the invention is the provision of a heat exchanger made from the anisotropic composition wherein the composition is constructed and arranged to provide increased transfer of heat between two mediums.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.